Expression des récepteurs activés par les proliférateurs de peroxysomes (PPAR) au niveau du colon sain et tumoral

Abstract : Peroxisome proliferator-activated receptors (PPAR) are transcription factors belonging to the nuclear receptor family. Three isotypes of PPAR have been described in humans (a, NUC-1 also called b or d; and g). PPAR have been implicated in a variety of biological processes including colon cancer. We have studied the expression of PPAR in colonic tissues in three biological situations: during development of the human fetal digestive tract, in Caco-2 cells used as a model of enterocyte-like differentiation and in biopsies from tumoral and normal adjacent human colon. The PPAR subtypes are expressed as early as 7 weeks of foetus development in cell types of endoderm and mesoderm origins. The presence of PPARg protein is found by Western blotting and that of the encoding mRNA by nuclease-S1 protection assay, confirming that this subtype is not adipocyte-specific. PPARa, PPARb and PPARg exhibit different spatio-temporal patterns of expression during morphogenesis of the digestive tract. Whatever the stage and the gut region (except the stomach) examined, PPARg is expressed at a high level, suggesting some fundamental role for this receptor in development and/or physiology of the human digestive tract. The expression of PPARa and g was next studied in Caco-2 cells. This cell line exhibits enterocyte-like differentiation during long term culture. We showed by immunohistochemistry that both isotype protein levels increased gradually during cell differentiation Using Nuclease S1 protection assay, we demonstrated that there is not a concomitant increase in the transcriptional level of PPAR subtypes, especially PPARa and isoforme PPARg2 which seemed to be regulated at the translational level. Since PPARg has been involved in human colon carcinoma, we finally investigated the expression of this PPAR subtype in biopsies obtained from human colon adenocarcinoma. At the transcriptional level, PPARg was less abundant in tumours than in normal-paired tissues. In contrast, using immunohistochemistry we showed that the PPARg protein amount was more abundant in tumoral tissues. In addition PPARg immunostaining was exclusively found in cytoplasm. Moreover, in normal adjacent tissues PPARg was present in the upper one-third of the crypts suggesting that PPARg expression is associated to the differentiation/maturation process of columnar cells.
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Cécile Huin. Expression des récepteurs activés par les proliférateurs de peroxysomes (PPAR) au niveau du colon sain et tumoral. Sciences agricoles. Université Henri Poincaré - Nancy 1, 2002. Français. ⟨NNT : 2002NAN10245⟩. ⟨tel-01754382⟩

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